Cellulose ethers have been used in various fields. For example, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC) or hydroxypropyl cellulose (HPC) have been widely employed in the fields of cosmetics, foods, engineering, coatings, and pharmaceuticals. On the other hand, it has been studied to introduce an additional substituent to a cellulose ether to develop a mixed cellulose ether having two different substituents and exhibiting new functions. Several mixed cellulose ethers have so far been put to practical use. For example, carboxymethylhydroxyethyl cellulose (CMHEC), which has been developed for improving salt resistance of CMC by introduction of a hydroxyethyl group, is used in petroleum boring. Hydroxypropylmethyl cellulose (HPMC), i.e., methyl cellulose having introduced therein a hydroxypropyl group, has a property of changing its solubility depending on pH and finds its use as a coating material for enteric-coated tablets. Mixed cellulose ethers are thus widely employed, and a number of processes for production thereof have been proposed.
In general, processes for preparing cellulose ethers include a process using an epoxy-containing oxide, e.g., ethylene oxide or propylene oxide, or a chlorohydrin type etherifying agent to obtain a hydroxyalkyl ether; a process using a chlorinating agent, e.g., monochloroacetic acid or methylene chloride; and a process using dimethyl sulfate. The process for obtaining a mixed ether varies depending on the combination of substituents to be introduced.
Where introduction of two or more substituents can be effected in the same reaction system, necessary reaction reagents are added to the system simultaneously or successively. On the other hand, where two or more substituents must be introduced in the respective system, a product of first introduction is once isolated and purified and then treated for introduction of a second substituent. While there are many proposals made to date with respect to production of mixed cellulose ethers, any of them achieved no more than a halfway improvement by a delicate alteration of reaction conditions, giving no essential solution to the above-described problem.
For example, JP-A-50-130883 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a process for producing a mixed cellulose ether having a dihydroxyalkyl group, in which an alkyl group or a hydroxyalkyl group is first introduced into cellulose to a degree of substitution of from 0.05 to 4.0 and the resulting cellulose ether is then reacted with, for example, glycidol to obtain a dihydroxypropyl-containing mixed cellulose ether. However, this process still needs further improvements in utilization of reagents and properties of an aqueous solution, such as clarity, of the resulting mixed ether.